Code division multiple access (CDMA) communication systems provide wireless communications between a base station and one or more mobile subscriber units. The base station is typically a computer-controlled set of transceivers that are connected to a land-based public switched telephone network (PSTN). The base station includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units. The base station antenna also receives reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for transmission of the reverse links signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a cellular modem. In CDMA cellular systems, multiple mobile subscriber units may transmit and receive signals on the same frequency, but with different modulation codes, to distinguish signals sent to or received from individual subscriber units.
The most common type of antenna for transmitting and receiving signals at a mobile subscriber unit is a monopole or omnidirectional antenna. This type of antenna consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. The transceiver receives reverse link signals to be transmitted from circuitry within the subscriber unit and modulates the signals onto a carrier signal at a specific frequency assigned to that subscriber unit. The modulated carrier signal is then transmitted by the antenna element. Forward link signals received by the antenna element at a specific frequency are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit.
The signal transmitted from a monopole antenna is omnidirectional in nature. That is, the signal is sent with substantially the same signal strength in all directions in a generally horizontal plane. Reception of a signal with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal in one azimuth direction versus detection of the same or a different signal coming from another azimuth direction. Generally, a monopole antenna does not produce significant radiation in the elevation direction. The antenna pattern is commonly referred to as a donut shape with the antenna element located at the center of the donut hole.
A second type of antenna that may be used by mobile subscriber units is described in U.S. Pat. No. 5,617,102. The system described therein provides a directional antenna comprising two antenna elements mounted on the outer case of a laptop computer, for example. The system includes a phase shifter attached to each element. The phase shifter may be switched on or off to effect the phase of signals transmitted or received during communications to and from the computer. By switching the phase shifters on and regulating the amount of phase shift imparted to the signals input thereto, the antenna pattern (which applies to both the receive and transmit modes) may be modified to provide a concentrated signal or beam in the selected direction. This is referred to as an increase in antenna gain or directionality. The dual element antenna of the cited patent thereby directs the transmitted signal into predetermined quadrants or directions to allow for changes in orientation of the subscriber unit relative to the base station, while minimizing signal loss due to the orientation change. In accordance with the antenna reciprocity theorem, the antenna receive characteristics are similarly effected by the use of the phase shifters.
CDMA cellular systems are also recognized as being interference limited systems. That is, as more mobile subscriber units become active in a cell and in adjacent cells, frequency interference becomes greater and error rates increase. As error rates increase, to maintain signal and system integrity, the operator must decrease the maximum data rates allowable to all users. In lieu of decreasing all user's data rates, the CDMA system operator can decrease the number of active mobile subscriber units, thus clearing the airwaves of potential interferers. For instance, to increase the maximum available data rate by a factor of two, the number of active mobile subscriber units is decreased by one half. However, this is rarely an effective mechanism to increase data rates because system users generally do not have priority assignments to be utilized in determining which users should be dropped from the system.